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AAPG Bulletin

Abstract


Volume: 49 (1965)

Issue: 4. (April)

First Page: 445

Last Page: 459

Title: General Theory of Epeiric Clear Water Sedimentation

Author(s): M. L. Irwin (2)

Abstract:

Shallow marine sedimentation is described in contrast to oceanic, coastal deposition. Shallow marine sedimentation occurs in non-oceanic epeiric seas where great width of shelves, their low order of slopes (less than one foot per mile), and extreme shallowness of the waters on them are sufficient in themselves to restrict or eliminate circulation. Oceanic, coastal deposition occurs where steep bottom slopes are the rule and where waves and tides extend to the shore.

Three marine, hydraulic energy zones, which are in effect environments of deposition, are thought to occur in epeiric seas: (1) a hundreds-of-miles wide, low-energy zone prevailing in the open sea beneath wave depth where marine currents are the only form of hydraulic energy acting upon the bottom (Zone X); (2) an intermediate, high-energy belt, tens-of-miles wide, beginning where waves first impinge upon the sea floor and thus expend their kinetic energy upon the bottom, extending landward to the limit of tidal action (Zone Y); (3) an extremely shallow, low-energy zone, tens- to hundreds-of-miles wide, occurring landward of Zone Y in which there is little circulation of water, where tides are essentially wanting, and in which the only wave action is that produced by local storms (Zon Z).

Reasons are given to show that in epeiric seas, which have received no invasion of terrigenous clastics ("clear water"), sediments formed in Zone Y are basically of biogenic origin, are sand-size or coarser, and are those most likely to become well-winnowed primary reservoirs; that those sediments formed in Zone Z are primarily fine-grained and tight, and are generally of chemical origin; that Zone X contains sediments which are mainly fine-grained detritus from Zone Y.

During a marine transgression, as each sedimentation zone migrates toward shore, it overlaps older sediments which earlier were deposited in an environment similar to the one then prevailing in the zone adjoining landward. In this manner, an ascending sequence of sediments, deposited under conditions of progressively less restricted circulation, the same as the pattern extending at a single time from land to sea, is established. During regression, each sedimentation zone migrates seaward, so that the ascending sequence of sediments is deposited under progressively more restricted conditions, and is the same as the one reaching at a single time from sea to land. Thus, in a regressive sequence, a rock type found on top of another would also occur landward from it; during transgression, ts position would be seaward. This process of continuous sedimentary overlap, as the sea advances and retreats, is the principle which explains cyclic sedimentation. It is noted that the age of geologic formations (lithofacies) becomes progressively younger in the direction of migration, emphasizing the necessity for time control.

Two examples are presented together with further discussion which points out how the regular sedimentary patterns may be altered by barriers (such as reefs), by climate, and by degree of bottom slope; and how an analysis of both the normal and altered patterns may be used for exploration and grading purposes.

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